Alzheimer's disease (AD) is a degenerative brain disorder characterized clinically by progressive memory deficits, confusion, gradual physical deterioration and, ultimately, death. Approximately 15 million people worldwide are affected by Alzheimer's disease, and the number is expected to increase dramatically as lifespan increases. Histologically, the disease is characterized by neuritic plaques, found primarily in the association cortex, limbic system and basal ganglia. The major constituent of these plaques is amyloid beta peptide (Aβ), which is the cleavage product of beta amyloid precursor protein (βPP or APP). APP is a type I transmembrane glycoprotein that contains a large ectopic N-terminal domain, a transmembrane domain, and a small cytoplasmic C-terminal tail. Alternative splicing of the transcript of the single APP gene on chromosome 21 results in several isoforms that differ in the number of amino acids.
Aβ appears to have a central role in the neuropathology of Alzheimer's disease. Familial forms of the disease have been linked to mutations in APP and the presenilin genes (Tanzi et al., 1996, Neurobiol. Dis. 3:159-168; Hardy, 1996, Ann. Med. 28:255-258). Diseased-linked mutations in these genes result in increased production of the 42-amino acid form of Aβ, the predominant form found in amyloid plaques. Moreover, immunization of transgenic mice that overexpress a disease-linked mutant form of APP with human Aβ reduces plaque burden and associated pathologies (Schenk et al., 1999, Nature 400:173-177; WO 99/27944), and peripheral administration of antibodies directed against Aβ also reduces plaque burden in the brain (Bard et al., 2000, Nature Medicine 6(8):916-919; WO 2004/032868; WO 00/72880).
It has been reported that Fc-mediated phagocytosis by microglial cells and/or macrophages is important to the process of plaque clearance in vivo. Bard et al., Proc. Natl. Acad. Sci. USA 100, 2023-2028 (2003). However, it has also been reported that non-Fc-mediated mechanisms are involved in clearance of amyloid-β in vivo by immunotherapy. Bacskai et al., J. Neurosci. 22:7873-7878 (2002); Das et al., J. Neurosci. 23:8532-8538 (2003).
Antibody therapy therefore provides a promising approach to the treatment and prevention of Alzheimer's disease. However, human clinical trials with a vaccine including Aβ1-42 were suspended due to meningoencephalititis in a subset of patients. Orgogozo et al., Neurology 61:7-8 (2003); Ferrer et al., Brain Pathol. 14:11-20 (2004). It has been reported that passive immunization with an N-terminal specific anti-Aβ antibody results in a significant reduction of mainly diffuse amyloid, but induces an increase of cerebral microhemorrhage frequency in transgenic mice that exhibit the age-related development of amyloid plaques and neurodegeneration as well as cerebral amyloid angiopathy (CAA) similar to that observed in the human AD brain. Pfeifer et al., Science 298:1379 (2002). It has been suggested that exacerbation of cerebral amyloid angiopathy (CAA)-associated microhemorrhage in APP transgenic mice by passive immunization with antibody directed to beta-amyloid is dependent on antibody recognition of deposited forms of amyloid beta peptide. Racke et al., J. Neurosci. 25:629-636 (2005). Passive immunization with antibodies against a peptide component of an amyloid deposit, which antibodies are devoid of Fc regions, has been suggested in order to decrease the risk of inflammation. WO 03/086310. There remains a need for antibodies and other immunotherapeutic agents directed against Aβ having improved efficacy and safety profile, and which are suitable for use with human patients.
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